1. Field of the Invention
This invention relates to an optical recording medium, in particular, an optical recording medium having a recording layer where a space charge field is designed to be formed by the irradiation of beam, and also to an optical recording apparatus which is designed to record information through such an optical recording medium.
2. Description of the Related Art
As a recording medium which is capable of recording data which requires a large memory capacity such as an image of high density, an optical recording medium is known to be useful. Conventionally, as an optical recording medium, a photomagnetic recording medium and an optical phase change recording medium have been developed. However, there is still increasing demands for an optical recording medium having a capacity for recording a more increased density of information.
There has been proposed a holographic memory as an optical recording medium for realizing the recording of such an increased density of information. In this holographic memory, a page data where an optical intensity, polarization, or the phase thereof is two-dimensionally modulated is interfered with a reference light beam so as to enable information to be stored as a hologram in a recording layer. This holographic memory is designed such that the thickness of the recording layer is made large so as to enable a large quantity of holograms to be recorded in the same overlapped region by slightly changing the incident angle of the reference light beam or by slightly displacing the recording position.
As for the materials useful for the recording medium of such a holographic memory, the employment of inorganic materials have been studied in the past. In recent years however, a photorefractive medium employing an organic polymer compound is now being extensively developed because of the reasons that it no longer necessitates to manufacture a crystalline material or that it is possible to obviate the difficulties in the control of characteristics of the crystal (For example, U.S. Pat. No. 5,064,264).
This photorefractive medium is provided with a recording layer containing a charge-generating material, a charge-transport material, a trapping material and a nonlinear optical material. As a signal beam and a reference beam, both beams interfering with each other, are simultaneously irradiated onto this recording layer, information is enabled to be recorded therein in the form of interference fringes of both beams. When the same reference beam that has been employed in the recording is irradiated onto the recording layer, a reconstructed beam having the same spatial characteristics as those of the signal beam is enabled to be read.
Specifically, in the portion of the recording layer that has been irradiated by the beams, an electric charge is generated from the charge-generating material and then isolated by the charge-transport material. By enabling this isolated electric charge to be retained by the trapping material, a space charge field is formed inside the recording layer. Due to this space charge field, the refractive index of the nonlinear optical material is caused to change. Therefore, when a pair of beams interfering with each other are irradiated onto the recording layer, an intensity pattern of the beams thus irradiated is enabled to be recorded, as a change in refractive index, in the recording layer.
However, when a difference in molecular orbital energy is increased between the trapping material and the charge-transport material so as to prevent the thermal elimination of recorded data due to the thermal de-trapping from the trapping material, it becomes increasingly difficult to permit the hopping of electric charge from the charge-transport material to the trapping material. The reason for this is explained by the charge transport theory based on the small polaron hopping (D. Emin, Adv. Phys. vol. 24, 305–347 (1975)). Therefore, it would take a long time for enabling an electric charge to be trapped by the trapping material, thus necessitating a long time for recording information. On the contrary, when an electric charge is enabled to be easily hopped from the charge-transport material to the trapping material, a difference in energy between the trapping material and the charge-transport material is caused to minimize, thereby enabling the thermal elimination of recorded data. Namely, the life of record would be shortened.
On the other hand, as an alternative optical recording medium for realizing the recording of high density of information, there is known a multi-layer optical recording medium (D. A. Parthnopulous and P. M. Rentzepis, Science vol. 245, pp. 843–844 (1989)). This optical recording medium is featured, as shown in this publication, in that a recording beam is converged at an optional portion within a uniform optical recording medium to bring about a change in optical characteristics only in the vicinity of the focused point, thus recording information. Therefore, this optical recording medium differs from those where a recording layer and a non-recording layer are alternately laminated.
In this multi-layer optical recording medium also, the employment of inorganic materials has been studied (Y. Kawata, H. Ishibashi, and S. Kawata, Opt. Lett. vol. 16, pp. 756–758 (1998)). In recent years however, a photorefractive medium employing an organic polymer compound is now being extensively developed, as mentioned above, because of the reasons that it no longer necessitates to manufacture a crystalline material or that it is possible to obviate the difficulties in the control of characteristics of the crystal. According to this photorefractive medium, the optical characteristics thereof changes in proportion to the intensity of beam. However, in a recording medium where the optical characteristics thereof changes in proportion to the intensity of beam, the recording region is expanded depthwise. Therefore, the recording regions neighboring to each other are required to be sufficiently spaced apart from each other, thereby obstructing the enhancement of densification of information. As a method for narrowing the distance depthwise between the recording regions, there has been studied a method of generating electric charge by two-photon absorption (D. Day, M. Gu, and A. Smallridge, Opt. Lett. vol. 24, pp. 288–290 (1999)). However, since this method requires a very strong light source for generating electric charge by two-photon absorption, an ordinary semiconductor laser can be no longer useful in performing the recording using this method.